A. Field of Invention
The present invention pertains to an electrostatic chuck used to hold conducting or semiconducting wafers (such as silicon wafers) in a low pressure or vacuum environment, so that they may be easily cooled or heated. In particular, the present invention relates to the arrangement of high quality materials used, the method of fabrication of a chuck and techniques for operation of a chuck according to the invention.
B. Description of Related Art
There are a number of semiconductor fabrication processes that have to be carried out in a low pressure environment (below one torr). Two that require wafer cooling are ion implantation and reactive ion etching. In both of these processes, poor thermal coupling exists between the chuck and wafer due to lack of thermal conduction through a gas between them. The ion bombardment that occurs in both implantation and etching leads to a high energy flux into the face of the wafer, and the wafer may overheat unless the heat can be removed by heat transfer to the chuck holding it. Using a cooled chuck will not solve the problem unless some way is found to improve the thermal coupling between the wafer and the chuck. A well-known way to do this is to hold the wafer with a clamp ring and add backside gas.
Other processes normally require wafer heating, rather than cooling. In this category are sputtering and most low pressure chemical vapor deposition processes. Again clamping and backside gas can be used to enhance the thermal coupling. In general, a wafer ring clamp is undesirable because a clamp covers part of the wafer surface. A preferred solution is to hold the wafer with electrostatic forces firmly enough to allow better thermal coupling for improved thermal transfer. This would avoid any need to touch the front of the wafer. In addition, the chuck should be fabricated from high temperature conductors and dielectrics to allow use at high temperature. The original concept for an electrostatic chuck was described by G. Wardly, "Electrostatic Wafer Chuck for Electron Beam Microfabrication," Rev. Sci. Instr. 44, 1506 (1973). The wafer acting as one plate of a capacitor was placed on a metal chuck covered with a dielectric. The metal chuck was then held at one electric potential, and the wafer was held at a different potential by contacting either its front or back surface. As is well known, there is a strong attractive force between the two plates of a capacitor when they are held at different potentials. Such an electrostatic chuck is referred to as monopolar. Many further developments have been reported in the patent literature. Approaches similar to Wardly's were described in Livesay (U.S. Pat. No. 3,983,401, Sep. 28, 1976) and McGinty (U.S. Pat. No. 3,993,509, Nov. 23, 1976).
Because the voltage is applied between the wafer and the electrode, electrical contact with the wafer is required. This requirement limits the wafers to be held to be conductors or semiconductors or to be coated with a conducting layer. For example, a silicon wafer coated with a layer of oxide (SiO.sub.2) cannot be held by a monopolar chuck. After Wardly's publication, Wachtler (U.S. Pat. No. 3,916,270, Oct. 28, 1975), Briglia (U.S. Pat. No. 4,184,188, Jan. 15, 1980) and Wicker (U.S. Pat. No. 4,724,510, Feb. 9, 1988) recognized that it was not necessary to contact the wafer if a split electrode concept is used, i.e., when the lower electrode of a capacitor is split into two equal parts and separated by an insulator, with each half placed at equal but opposite voltages (+V and -V), then the upper electrode (the wafer) must be at ground potential (V=0) because of symmetry. In this case, there is a potential difference between each half of the lower electrode and its respective portion of the upper electrode (the wafer) of V. The attractive force between two plates of a capacitor depends on the voltage difference squared, so there are equal holding forces on each wafer half. Most importantly, it is not necessary to touch the wafer surface in order to maintain the voltage at the wafer at a selected (preferably zero) potential relative to ground. Such electrostatic chucks are referred to as bipolar.
Wachtler, Briglia and Wicker all described an "interdigitated" bipolar arrangement that is fairly difficult to fabricate. A simpler arrangement was described by Abe (U.S. Pat. No. 4,384,918 May 24, 1983), wherein each electrode is a simple half circle. Fabricating such a bipolar chuck presents many practical difficulties. The force of attraction is very sensitive to flatness of the chuck surface, and it is difficult to electrically isolate each half electrode. A further refinement on the bipolar concept was described in Suzuki (U.S. Pat. No. 4,692,836, Sep. 8, 1987). He pointed out that by using a radially segmented bipolar design, a wafer that is initially bowed up in the center can be more easily flattened by activating the central electrodes first. Unfortunately, practical semiconductor wafers are more likely to be warped (somewhat like a potato chip) rather than simply bowed. The Suzuki design offers no solution to this problem.
Even though an electrostatic chuck can hold a wafer in a low pressure reactor, there may still be difficulty in transferring heat into or out of the wafer. When the gas pressure between the wafer and the chuck is less than 10 torr, there is very little heat conduction through the gas, so heat has to be transferred by radiation and conduction through the few points of solid-to-solid contact. Experience has shown that a 500.degree. C. chuck temperature will produce a 400.degree. C. wafer temperature in a low pressure environment. This will not allow adequate control for critical semiconductor processes. Lewin (U.S. Pat. No. 4,502,094, Feb. 26, 1985) suggested the use of thermally conducting spacers between the conducting grid lines of an interdigitated bipolar design. Logan (U.S. Pat. No. 5,055,964 Oct. 8, 1991) described an aluminum chuck coated with aluminum oxide. The problem with both of these approaches is undesirably high solid-to-solid thermal contact resistance at low pressures. Moreover, the invention of Logan is limited to temperatures below 450.degree. C. because of its aluminum construction. Tezuka (U.S. Pat. No. 4,771,730, Sep. 20, 1988) proposed a solution to the problem of chuck-to-wafer heat transfer by introducing backside gas. However, consideration was limited to an aluminum electrode with aluminum oxide coating or a plastic dielectric. In either case, the chuck cannot be used for high temperature operation (e.g., greater than 450.degree. C.). Ward (U.S. Pat. No. 4,665,463, May 12, 1987) solves the contact problem just described by using a plastic dielectric loaded with a thermally conducting powder. This approach relies on the improved thermal contact between a compliant plastic and the hard wafer surface. However, a plastic dielectric cannot be used for high temperature applications (e.g., greater than 200.degree. C.). In addition, Ward notes that his device has a retention characteristic. That is, after a wafer has been held, it cannot be released for as long as 24 hours. Such wafer retention, even for times much less than 24 hours, poses obvious difficulties in practical reactor operation.
The problem of wafer retention or sticking was dealt with in Horowitz (W088/0905), where an AC chuck was described. When a DC voltage is applied, the dielectric separating the wafer and the metal chuck can become permanently polarized and, after the voltage is removed, the residual polarization can hold a wafer for some time, as noted above. Horowitz suggested the use of sapphire or boron nitride as the dielectric material because of their ability to transfer RF power efficiently. However, he did not describe any method of fabricating such a chuck. In particular, when two pieces of a single crystal material are joined by a high temperature process, it is important to know whether or not the crystalline material is anisotropic. In other words, when heated, it may expand different amounts in different crystal orientations. In this case, when the joined parts cool to room temperature, the assembly warps. It should be noted that large diameter discs or wafers of sapphire or boron nitride as required by the Horowitz technique are prohibitively expensive. At the same time, AC excitation of an electrostatic chuck introduces many practical difficulties in designing and operating such a system.
Another approach to dealing with the problem of slow wafer release is described in U.S. Pat. No. 5,117,121, May 26, 1992. Watanabe describes a chuck made of ceramic that is inherently susceptible to retention forces. He proposed solving the problem by applying a high voltage reverse bias (1.5.times.-2.times.) when release is desired. This high voltage increases the risk of breakdown of the dielectric and, in addition, is hard to control in a practical circuit. For example, if the reverse bias is held too long, the wafer will stick again and will not release. Accordingly, there is a need for a more practical, more reliable and less expensive mechanism for holding and reliably releasing wafers in a vacuum environment.